US Secretary of Defense James Mattis on Friday said there was "no longer any doubt" that Syrian President Bashar al-Assad's regime still has chemical weapons.

During a joint press conference with Israeli Defense Minister Avigdor Lieberman in Tel Aviv, Mattis said Assad's regime retains chemical weapons, though he did not provide evidence. The Assad regime is accused of carrying out a chemical attack April 4 in the rebel-held town of Khan Sheikhoun in the Idlib province. More than 80 people died and hundreds others were injured.

"There can be no doubt in the international community's mind that Syria has retained chemical weapons in violation of its agreement and its statement that it had removed them all. There is no longer any doubt," Mattis told reporters.

In response to accusations Assad's regime used chemical weapons on civilian populations, US President Donald Trump ordered a missile attack on April 6 that Mattis said destroyed about 20 percent of Syria's operational aircraft.

The missile strikes were the first known direct US attack on the Syrian government since the country's civil war began in 2011. Though Mattis did not present evidence corroborating allegations about Assad's chemical stockpile, Mattis asserted "authoritatively" that Assad's regime still had chemical weapons.

"The bottom line is, I can say authoritatively they have retained some [chemical weapons]. It's a violation of the United Nations Security Council resolutions, and it's going to have to be taken up diplomatically," Mattis said.

Meanwhile, evacuations restarted Friday after a two-day pause as part of the "Four Towns Agreement" between the Syrian regime and rebel groups. The warring sides once again began busing thousands of people out of four besieged towns.

Civilians in the government-held northwestern towns of Foah and Kefraya are being bused out, while civilians living in two rebel-held towns near Damascus, Madaya and Zabadani, will also be given safe passage. More than 30,000 people in total are expected to be moved in the efforts that began April 13.

On Saturday, a convoy of buses full of civilians evacuating from Foah and Kefraya was targeted in a bombing in which at least 126 people died. The Syrian Observatory for Human Rights said evacuation efforts stopped because of rebel demands that the regime release 750 prisoners.